Micro-capsules comprising a capsule core containing water-soluble substances

ABSTRACT

Microcapsules having a capsule core comprising water-soluble organic substances, and a capsule coating which is a condensate of formaldehyde resins and/or alkyl ethers thereof, a process for their preparation, their use, and compositions comprising the microcapsules.

The present invention relates to microcapsules having a capsule corecomprising water-soluble organic substances, and a capsule coating whichis a condensate of formaldehyde resins and/or alkyl ethers thereof, to aprocess for their preparation, to their use, and to compositionscomprising the microcapsules according to the invention.

Microcapsules based on melamine-formaldehyde resin having core oilscomprising color formers have been known for a long time in the field ofcopy papers (see e.g. EP-A-0 026 914 and EP-A-0 218 887). They areobtained by polycondensation of the resins of an oil-in-water emulsionthat are located in the aqueous phase.

Also known, from U.S. Pat. No. 3,418,656, are microcapsules withmelamine-formaldehyde resin condensates as wall material and an emulsionas core.

WO 91/10506 teaches microcapsules with an aqueous core, whose walls formby the reaction of a water-soluble Lewis acid, such ascarboxymethylcellulose, with a Lewis base dissolved in the lipophilicphase, such as benzalkonium chloride. The materials encapsulated aremedicinal active ingredients which, since the capsule walls are notsealed, are released in a delayed manner.

JP-60-244336 teaches microcapsules with walls ofhexamethoxyhexamethylol-melamine resin. The hydrophobic melamine resinis polymerized in the core oil of the oil-in-water emulsion.

In principle, dye-containing polymers can be prepared by two differentmethods. One procedure consists in homogeneously dissolving dyes whichhave sufficient solubility in polymers, if necessary at elevatedtemperature, in said polymers. The advantage here is, in particular, ahigh brilliance of the color impression and a high color intensity, i.e.a low dye requirement. However, this procedure has the disadvantage thatthe dyes migrate on heating or when the polymers come into contact withsolvents; this causes reduced weather and migration fastnesses since dyemolecules which have migrated to the surface of the substrate areattacked by light or oxygen or are flushed out by solvents. The otherprocedure consists in distributing insoluble color pigments in thepolymer matrix in a dispersed manner. This avoids the disadvantageswhich may arise when the colorant is homogeneously dissolved in thepolymer matrix. A disadvantage of this procedure, however, is that theindividual dye molecules interact with one another in the pigmentcrystals, leading to a broadening of the absorption bands and thus to anundesired decrease in the purity of shade and color intensity of thecoloration. Furthermore, the shape anisotropy of the pigmentcrystallites gives rise to rheology problems, such as thixotropies, onincorporation into the polymeric matrix.

The disadvantages of both procedures can, in principle, be circumventedby replacing the color pigments by dye-containing polymer dispersions inwhich the dye is distributed homogeneously in the polymer particles.Such “pigments” should on the one hand be characterized by a high colorbrilliance and purity of shade and should on the other hand becharacterized by low rheology problems, owing to the spherical shape ofthe polymer particles.

WO 99/40123 describes aqueous polymer dispersions which are prepared byemulsion polymerization of dye-containing monomer solutions in aqueousphase.

Dyes used for polymer dispersions are hydrophobic since they have to bedissolved in the monomers or in the polymer. However, this limits thechoice of dyes available.

It is an object of the present invention to provide organicwater-soluble substances, such as dyes, in a form in which they behaveinertly toward solvents.

We have found that this object is achieved by the above-describedmicrocapsules.

The capsules comprise a capsule coating and a capsule core. The capsulecore comprises at least one water-soluble organic substance as solidand/or, depending on the preparation, as solution in a hydrophilicsolvent. Preferred capsule cores are solutions of the water-solubleorganic substance.

The basic principle of capsule formation is that the formaldehyde resindissolved in the hydrophilic solvent and which is the hydrophilic phaseof a water-in-oil emulsion becomes insoluble over the course of thecondensation and migrates to the interface of the hydrophobic phase,where it forms the capsule wall.

Hydrophilic solvent is to be understood as meaning either water or thoseaqueous solutions which, apart from water, comprise up to 20% by weightof a water-miscible organic solvent, such as C₁- to C₄-alkanols, inparticular methanol, ethanol, isopropanol or a cyclic ether, such astetrahydrofuran.

Suitable hydrophilic solvents are also ethylene glycol, glycerol,polyethylene glycols and butylene glycol, and their mixtures. Preferredhydrophilic solvents are water and mixtures of these solvents withwater.

Suitable hydrophobic phases of the water-in-oil emulsion are solventswith only limited miscibility with water so that phase separation takesplace. Hydrophobic solvents which may be used are virtually allwater-immiscible liquids which do not interfere with thepolycondensation, i.e. are inert. Solvents suitable according to theinvention are preferably solvents with a solubility in water at 20° C.of ≦65 g/l. Preference is given here to using aliphatic and aromatichydrocarbons or their mixtures. Suitable aliphatic hydrocarbons are, forexample, pentane, hexane, heptane, octane, nonane, decane, cyclohexane,decalin, methylcyclohexane, isooctane and ethylcyclohexane. Suitablearomatic hydrocarbons are, for example, benzene, toluene, xylene andisopropylbenzene. In addition, it is also possible to use halogenatedhydrocarbons, such as tetrachloroethane, hexachloroethane,trichloroethane and chlorobenzene. In addition, aliphatic esters, suchas ethyl acetate, are suitable. Preference is given to using aliphatichydrocarbons and, in particular, cyclohexane. Preference is given tosolvents whose boiling point is ≦120° C. since these solvents can beremoved advantageously if the microcapsules are dried.

A water-soluble organic substance is to be understood as meaning acompound based on carbon which is at least partially soluble in water.The organic substance must have greater affinity to the hydrophilicphase than to the hydrophobic phase. This is generally ensured if thesubstance has a solubility in the hydrophilic solvent at roomtemperature of at least 1 g/l. The organic substances preferably have asolubility in the hydrophilic solvent of ≧20 g/l.

The water-soluble, organic substances are, for example, water-solubledyes.

The term “dye” includes here and below organic compounds or salts oforganic compounds, and charge transfer complexes of organic compoundscontaining a chromophore which has an absorption maximum in thewavelength range from 400 to 850 nm and thus gives rise to a colorimpression for the human eye (conventional dyes) and which itself mayalso emit light in the visible region (fluorescent dyes). For thepurposes of this invention, dyes are also compounds with an absorptionmaximum in the range from 250 to 400 nm which, upon irradiation with UVlight, emit fluorescent radiation in the visible region (opticalbrighteners). For the purposes of this invention, dyes are also organiccompounds which absorb light of wavelength <400 nm and deactivate it ina nonradiative manner (UV stabilizers).

The water-soluble dyes usually have ionic functional groups whichimprove the solubility in the aqueous solvent. In this connection, themodification can be carried out cationically or anionically. Suitablesubstituents are, for example, sulfonic acid, carboxylic acid andphosphoric acid radicals, and also ammonium and alkylammonium radicals.

Dyes suitable according to the invention include a variety of classes ofdyes having various chromophores, for example monoazo and disazo dyes,triarylmethane dyes, metal complex dyes, such as phthalocyanine dyes,quinophthalones and methine and azamethine dyes.

By way of example, reference may be made to the following Colour Indexnumbers:

Direct Yellow 4, 5, 11, 50, 127, 137, 147, 153; Acid Orange 7, 8; DirectOrange 15, 34, 102; Direct Red 81, 239, 252–255; Direct Violet 9, 51;Acid Blue 9, 86; Direct Blue 199, 218, 267, 273, 279, 281; Acid Black194, 208, 210, 221; Direct Black 19, 161, 170 and 171;

Basic Red 1, Basic Red 14, Basic Blue 7, Basic Blue 11, Basic Blue 26,Basic Violet 1, Basic Violet 4, Basic Violet 10 etc.

The dyes also include complexes of basic and acidic dyes and complexesof anionic and cationic dyes, for example the complex of chrysoidinebase and metanil yellow acid.

According to the invention, the dyes also include optical brightenerswhich are at least partially soluble in water.

In accordance with the definition, the organic dyes also includeUV-ray-absorbing compounds (UV stabilizers) which deactivate theabsorbed radiation in a nonradiative manner. Such compounds arefrequently used as UV absorbers in sunscreen compositions. These includederivatives of p-aminobenzoic acid, in particular its esters;salicylates, cinnamates, benzophenones, 2-phenylbenzimidazole-4-sulfonicacid and salts thereof, urocanic acid, salts thereof and esters thereof,benzoxazoles, benzotriazoles, benzylidenecamphor and its derivatives.

Depending on the color intensity of the dye, the microcapsule usuallycomprises at least 0.1% by weight, based on the hydrophilic solvent,preferably 1 to 50% by weight and in particular 5 to 20% by weight, ofat least one dye.

The capsule coating according to the invention is a condensate offormaldehyde resins and/or alkyl ethers thereof. Formaldehyde resinsare, for example, reaction products of formaldehyde with

-   -   triazines, such as melamine    -   carbamides, such as urea    -   phenols, such as phenol, m-cresol and 3,5-xylene    -   amino and amido compounds, such as aniline,        p-toluenesulfonamide, ethyleneurea and guanidine,        or their mixtures.

Preferred formaldehyde resins are urea-formaldehyde resins,urea-resorcinol-formaldehyde resins, urea-melamine resins andmelamine-formaldehyde resins. Also preferred are the C₁–C₄-alkyl, inparticular methyl, ethers of these formaldehyde resins, and the mixtureswith these formaldehyde resins. Particular preference is given tomelamine-formaldehyde resins and/or methyl ethers thereof. A preferredstarting material for the wall material are melamine-formaldehyde resinsand/or methyl ethers thereof, with a ratio of melamine:formaldehyde offrom 1:1.5 to 1:8 in the resin, preferably 1:3 to 1:6. These resins areN-methylolmelamine compounds or methyl ethers thereof. The resins usedfor the process according to the invention must be miscible with thehydrophilic solvent in any ratio without producing clouding. For thesereasons, partially etherified methylolmelamines are particularlypreferred.

The present invention also covers a process for the preparation of themicrocapsules having a capsule core comprising water-soluble organicsubstances, and a capsule coating, by condensing formaldehyde resins inthe hydrophilic phase of a water-in-oil emulsion in the presence of aprotective colloid.

In order to obtain a stable water-in-oil emulsion, surface-activesubstances such as protective colloids are required. Such protectivecolloids are known from processes for inverse suspension polymerization,cf. DE-A-1081228 and DE-A-3709921. Use is usually made of protectivecolloids which dissolve in the hydrophobic phase.

Examples of suitable hydrophobic protective colloids are alkylatedpolyvinylpyrrolidones, ethylene oxide/propylene oxide copolymers andethoxylated fatty alcohols (EO degree=3 to 50, alkyl radical: C₈ toC₃₆).

Preference is given to hydrophobic protective colloids based oncopolymers of monoethylenically unsaturated carboxylic acids withmonovinylaromatic comonomers, e.g. styrene, esters of (meth)acrylicacid, vinyl acetate, acrylamide, methacrylamide, acrylonitrile andhydroxyalkyl (meth)acrylates.

Protective colloids which have proven particularly advantageous arethose obtainable by grafting polymers A) comprising

-   -   a) 40 to 95% by weight of monovinylaromatic monomers,    -   b) 5 to 60% by weight of monoethylenically unsaturated        carboxylic acids having 3 to 6 carbon atoms, maleic anhydride        and/or itaconic anhydride and    -   c) 0 to 20% by weight of other monoethylenically unsaturated        monomers,    -   with the proviso that the sum of the percentages by weight (a)        to (c) is always 100 and the polymers (A) have a molecular        weight (number-average) of from 500 to 20 000, with monomer        mixtures comprising    -   a) 70 to 100% by weight of acrylic esters and/or methacrylic        esters of monohydric alcohols containing 1 to 20 carbon atoms,    -   b) 0 to 15% by weight of monoethylenically unsaturated        carboxylic acids having 3 to 6 carbon atoms, maleic anhydride        and/or itaconic anhydride,    -   c) 0 to 10% by weight of acrylic monoesters and/or methacrylic        monoesters of at least dihydric alcohols,    -   d) 0 to 15% by weight of monovinylaromatic monomers and    -   e) 0 to 7.5% by weight of acrylamide and/or methacrylamide with        the proviso that the sum of the percentages by weight a) to e)        is always 100,        where the monomers are used in an amount of from 97.5 to 50% by        weight, based on the mixture of polymer (A) and monomers.

Such graft polymers and processes for their preparation are known fromDE-A-3709921.

The polymers A) are copolymers of monovinylaromatic monomers with anethylenically unsaturated carboxylic acid or an anhydride of amonoethylenically unsaturated carboxylic acid.

Suitable as component a) of the polymers A) are monovinylaromaticmonomers, e.g. styrene, α-methylstyrene, α-alkylstyrenes having 2 to 6carbon atoms in the alkyl radical, which may be straight-chain and/orbranched, e.g. α-isobutylstyrene. Also suitable are vinylaromaticswhich, apart from the vinyl group on the aromatic core, carry a C₁- toC₈-alkyl group, e.g. vinyltoluene, tert-butylstyrene, halogenatedstyrenes, core (alkyl)-substituted α-alkylstyrenes having 1 to 8 carbonatoms in the core alkyl radical and having 1 to 6 carbon atoms in theα-alkyl radical, e.g. para-tert-butyl-α-methylstyrene. Preference isgiven to using styrene from this group of monomers. Components a) inparticular styrene are preferably involved in an amount of 60 to 95% byweight in the construction of the polymer A).

The monomers of group b) include monoethylenically unsaturatedcarboxylic acids having 3 to 6 carbon atoms and/or anhydrides thereof,e.g. acrylic acid, methacrylic acid, ethacrylic acid, maleic acid,fumaric acid, itaconic acid, maleic anhydride, itaconic anhydride,vinyllactic acid, vinylphosphonic acid and vinylsulfonic acid. Thesemonomers are either used alone or in a mixture. From this group ofmonomers, preference is given to using acrylic acid, methacrylic acid,maleic anhydride and itaconic anhydride. The monomers of this group arepreferably involved in an amount of 5 to 40% by weight in theconstruction of the polymers A).

Apart from the monomers of groups a) and b), up to 20% by weight ofother monoethylenically unsaturated monomers may also be present in thepolymers A) in polymerized form. This group of monomers includes, forexample, the esters of acrylic acid, methacrylic acid and/or ethacrylicacid which are derived from alcohols having 1 to 12 carbon atoms, e.g.methyl acrylate, methyl methacrylate, tert-butylcyclohexyl acrylate,tert-butylcyclohexyl methacrylate, cyclohexyl acrylate, cyclohexylmethacrylate, tert-butyl acrylate, tert-butyl methacrylate, n-butylacrylate, n-butyl methacrylate, isobutyl acrylate, isobutylmethacrylate, 2-ethylhexyl acrylate, 2-ethylhexyl methacrylate, andvinyl esters of saturated aliphatic carboxylic acids which contain 2 to10 carbon atoms, e.g. vinyl acetate, vinyl propionate, vinyl laurate,vinyl butyrate and vinyl stearate. Another group of monomers are theamides of methacrylic acid, acrylic acid and ethacrylic acid. Here,these may, for example, be methacrylamide or acrylamide, andN-substituted amides, such as N-tert-butylmethacrylamide orN-tert-butylacrylamide. A further class of monomers which may beinvolved in the construction of the polymer A) are acrylonitrile andmethacrylonitrile. The monomers of the group of c) can either beincorporated by polymerization into the copolymer A) alone or in amixture in amounts up to 20% by weight. The sum of the percentages a),b) and c) is 100 in each case.

The polymers A) have a molecular weight (number-average) of from 500 to20 000 and hydrogenation iodine numbers (in accordance with DIN 53241)of from 1.3 to 51, preferably 2.5 to 25.4. It is particularly preferredto use polymers A) whose hydrogenation iodine numbers are 5.1 to 16.9.The average molecular weight (number-average) of the polymers A) ispreferably 1 000 to 10 000. Preference is given to using those polymersA) which are sparingly soluble in aliphatic hydrocarbons with a boilingrange from 50 to 150° C.

Such polymers are known. They are prepared, for example, by homo- orcopolymerization of the monomers a) to c) without diluent at 180 to 400°C., preferably 200 to 300° C. Particular preference is given tocontinuous bulk polymerization of the monomers which is carried out inthe given temperature range and, in particular, at 200 to 260° C. and atpressures of from 1 to 100 bar, preferably 20 to 50 bar, in the absenceof polymerization initiators or else in the presence of polymerizationinitiators and polymerization inhibitors. Such processes are described,for example, in DE-A-3026831, DE-A-3046476, U.S. Pat. No. 4,042,768 andWO 82/2387.

The polymers A) serve as graft base for the preparation of theprotective colloids. For the preparation of the graft polymers, theprocedure generally involves adding some of the polymerization initiatorand some of the monomer mixture to a solution or dispersion of thepolymers A) in an aliphatic and/or aromatic hydrocarbon—preference isgiven to using the same solvents which are subsequently used in thepreparation of the microcapsules as hydrophobic inert solvents—andheating to an elevated temperature and, after the polymerization hasstarted, adding the remaining monomers and the polymerization initiator.

The monomers of group a) used are acrylic esters and/or methacrylicesters of monohydric alcohols containing 1 to 20 carbon atoms. Examplesof this group of monomers are methyl acrylate, methyl methacrylate,ethyl acrylate, ethyl methacrylate, propyl acrylate, isopropyl acrylate,propyl methacrylate, n-butyl acrylate, sec-butyl acrylate, tert-butylacrylate, tert-butyl methacrylate, sec-butyl methacrylate, n-butylmethacrylate, tert-butylcyclohexyl acrylate, tert-butylcyclohexylmethacrylate, cyclohexyl acrylate, 2-ethylhexyl acrylate, 2-ethylhexylmethacrylate and lauryl acrylate. It is also possible to use the estersof ethacrylic acid which are derived from monohydric alcohols having 1to 12 carbon atoms. From this group of monomers, preference is given tousing acrylic esters and/or methacrylic esters of monohydric alcoholscontaining 3 to 6 carbon atoms. Very particular preference is given tothe use of tert-butyl acrylate, n-butyl acrylate and isobutyl acrylateor mixtures thereof in an amount of from 85 to 98% by weight, based onthe monomer mixture which is grafted onto the polymer A). The monomersof group a) are generally used in an amount of from 70 to 100% byweight, based on the monomers to be grafted. In the case of only aslight modification, the amount of monomers of group a) is 92.5 to 100%by weight.

Suitable monomers of group b) are monoethylenically unsaturatedcarboxylic acids having 3 to 6 carbon atoms and/or anhydrides thereof.This group of monomers corresponds to the monomers of group b) ofpolymers A). Specifically, these are, for example, acrylic acid,methacrylic acid, ethacrylic acid, fumaric acid, the anhydrides of saidcarboxylic acids and maleic anhydride. This group of monomers isoptionally co-used in the preparation of the graft polymers and ispresent in an amount of from 0 to 15% by weight in the monomer mixturewhich is grafted onto the polymer A). From this group of monomers,preference is given to using acrylic acid, methacrylic acid, maleicanhydride or itaconic anhydride in an amount of from 0 to 7.5% byweight, based on the monomers.

A further group of monomers which can be grafted onto the polymer A)which may be mentioned are acrylic monoesters and/or methacrylicmonoesters of at least dihydric alcohols. These include, for example,hydroxyethyl acrylate, hydroxyethyl methacrylate, hydroxypropylacrylate, hydroxypropyl methacrylate, hydroxybutyl acrylate andhydroxybutyl methacrylate. This group of monomers is optionally co-usedin a monomer mixture in an amount up to 10% by weight.

A further modification of the polymers A) can take place by grafting onmonomers of group d). These monomers include monovinylaromatic compoundswhich may be present in an amount up to 15% by weight, preferably 1 to7.5% by weight, in the monomer mixture. These monomers are identical tothe monomers of group a) of polymers A). From this group of monomers,preference is given to using styrene.

A further modification can take place if the mixture of the monomerswhich are grafted onto the polymers A) comprises, as monomers e),optionally up to 7.5% by weight of acrylamide and/or methacrylamide.

The sum of the percentages by weight of the monomers of group a) to e)is always 100. The monomers a) to e) are used in an amount of from 97.5to 50% by weight, preferably 90 to 75% by weight, based on the mixtureof polymer A) and the monomers a) to e), for the preparation of thegraft polymers.

The graft polymerization is generally carried out at temperatures up to50° C., preferably from 50 to 150° C., preferably 60 to 120° C., in thepresence of polymerization initiators, which are generally used in anamount of from 0.01 to 6% by weight, preferably 0.1 to 4% by weight,based on the weight of the polymers A) and the monomer mixture. Thegraft polymerization can be carried out at atmospheric pressure, andalso at elevated or reduced pressure.

Polymerization initiators for the graft polymerization are known and aregiven, for example, in DE-A-3709921.

The K value according to Fikentscher (Cellulose Chemie, vol. 13, 48–64and 71–74 (1932)) at 25° C. in a 1% strength by weight solution intetrahydrofuran, of the graft copolymers to be used as protectivecolloids is 25 to 100, preferably 34 to 65.

The optimum amount of protected colloid is influenced firstly by theprotective colloid itself, and secondly by the reaction temperature, thedesired microcapsule size and the formaldehyde resin mixture. Simpleseries of experiments can readily determine the optimum amount required.To prepare the water-in-oil emulsion, the protective colloid isgenerally used in an amount of from 5 to 30% by weight, based on thehydrophobic phase.

The process for the preparation of the microcapsules according to theinvention is generally carried out by emulsifying a mixture comprisingthe water-soluble organic substance and the formaldehyde resin and/oralkyl ethers thereof with the hydrophilic solvent in the hydrophobicsolvent comprising the protective colloid to give fine droplets, itbeing possible to adjust the droplet size depending on the intendedapplication purpose.

The condensation of the formaldehyde resin is carried out with acidiccatalysis and is accelerated by increasing the temperature. In general,the condensation is carried out in a temperature range from 40 to 150°C. A pH of the aqueous solvent is generally chosen in the range from 3to 6.5. The temperature during the formation of the water-in-oilemulsion is usually 20 to 45° C. In this temperature range, thepolycondensation proceeds only very slowly, if at all. The capsulecoating is then fully cured by increasing the temperature. Curing takesplace at varying rates depending on the pH of the dispersion. In the pHrange from 3 to 5, the polycondensation preferably takes place attemperatures up to 100° C., in particular 85° C. In a preferredembodiment, the temperature is increased continuously or stepwisestarting at 40° C. to 100° C., preferably to 85° C. Where appropriate,heating is then carried out to temperatures up to 150° C. in order toachieve post-crosslinking. The optimum temperatures, depending on thepH, can be readily determined by simple experimental series.

The pH in the aqueous phase can be adjusted using acids, preferablyusing formic acid.

Dispersion of the core material is carried out in a known mannerdepending on the size of the capsules to be prepared. For thepreparation of large capsules, dispersion using effective stirrers, inparticular propeller or impeller stirrers, suffices. Small capsules,particularly if the size is to be below 50 μm, require homogenizers ordispersion machines, with or without forced-flow means.

The homogenization can also be carried out using ultrasound (e.g.Branson Sonifier II 450). For homogenization by means of ultrasound,suitable equipment is, for example, that described in GB 2250930 andU.S. Pat. No. 5,108,654.

The capsule size can be controlled via the speed of the dispersionapparatus/homogenization apparatus and/or using the concentration of thepolymers carrying sulfonic acid groups or via the molecular weightthereof, i.e. via the viscosity of the aqueous continuous phase, withincertain limits. Here, as the speed increases up to a limiting speed, thesize of the dispersed particles decreases.

In this connection, it is important that the dispersion apparatuses areused at the start of the capsule formation. In the case of continuouslyoperating apparatus with forced-flow, it is advantageous to pass theemulsion through the shear field a number of times.

The conditions optimum for individual cases, such as temperature, pH andstirrer speed, can be readily determined by a few experiments.

Using the process according to the invention it is possible to preparemicrocapsule dispersions with a content of from 15 to 60% by weight ofmicrocapsules. The microcapsules are individual capsules. If suitableconditions are chosen during the dispersion it is possible to preparecapsules with an average particle size in the range from 0.5 to 50 μmand above. Preference is given to capsules with an average particle sizeof from 0.5 to 50 μm, in particular up to 30 μm. The average particlediameter is the number-average particle diameter, determined byquasi-elastic dynamic light scattering. It is usually determined using aCoulter N4 Plus Particle Analyzer from Coulter Scientific Instruments.The size distribution of the capsules is particularly advantageouslyvery narrow.

Despite the high concentration of capsules, the capsule dispersions havea very low viscosity and can therefore also be filtered rapidly throughmachine sieves having a mesh size of from 25 to 40 μm. During thefiltration, it is found that the yield of microcapsules in the processaccording to the invention is very high.

The capsule dispersion obtained according to the invention can befurther processed directly. For example, it can be incorporated intoplastic molding compositions.

According to the invention, the microcapsules can also be used aspowders. The preparation is generally carried out by spray-drying,optionally in the presence of spray auxiliaries, in a stream of warmair, or by freeze-drying. Processes for spray-drying and freeze-dryingare known in principle to the person skilled in the art and can betransferred to the drying of the above-described microcapsuledispersion.

In the case of spray-drying, the procedure involves, for example,spraying the microcapsule dispersions to be dried in a customary dryingtower in a stream of warm air. Here, the inlet temperature of the streamof warm air is in the range from 100 to 200° C., preferably 120 to 160°C., and the outlet temperature of the stream of warm air is in the rangefrom 30 to 90° C. and preferably 60 to 80° C. The spraying of themicrocapsule dispersion in the stream of warm air can be carried out,for example, using single-component or multi-component nozzles or via arotating disk. The microcapsule powders are normally separated off usingcyclones or filter separators. The sprayed microcapsule dispersion andthe stream of warm air are preferably introduced in parallel.

Suitable spray auxiliaries, which are also referred to as dryingauxiliaries, are neutral, cationic, anionic or amphoteric, water-solublepolymers. They generally have a molecular weight M_(N) in the range from1 000 to 1 000 000, preferably 2 000 to 100 000.

Specific examples of neutral polymers are: polyvinyl alcohols (see e.g.EP-A-56 622, EP-A-680 993, DE-A-22 14 410 and DE-A-26 14 261),polyvinylpyrrolidones (see e.g. DE 22 38 903 and EP 576 844). Examplesof anionic polymers are phenolsulfonic acid/formaldehyde condensates(see e.g. EP-A 407 889, WO 98/03576), naphthalenesulfonicacid/formaldehyde condensates (see e.g. WO 98/03577), homo- andcopolymers of 2-acrylamido-2-methylpropanesulfonic acid (see e.g. EP-A629 650, EP-A 671 435 and DE-A 195 39 460), copolymers of ethylenicallyunsaturated carboxylic acids, such as, in particular, acrylic acid,methacrylic acid and maleic acid, with hydrophobic comonomers, such asstyrene (see e.g. EP 467 103) or olefins (see e.g. EP 9 169) or withhydroxyalkyl esters (see e.g. JP 59 162 161). Examples of cationicpolymers are copolymers and terpolymers of vinylpyrrolidone and/or ofvinylcaprolactam with 1-vinyl-3-alkylimidazolinium salts, e.g. with1-vinyl-3-methylimidazolinium chloride or methosulfate; copolymers andterpolymers of vinylpyrrolidone and/or of vinylcaprolactam with(meth)acryloyloxyethyltrialkylammonium salts or with(meth)acryloyloxyethylammonium salts. Such cationic polymers are knownto the person skilled in the art and are available commercially.

Suitable amphoteric polymers are copolymers of acrylic acid andoptionally hydrophobic monomers such as styrene and optionallywater-soluble, neutral monomers with cationic monomers, e.g. copolymersof acrylic acid with styrene and with(meth)acryloxyethyltrialkylammonium salts, and optionally with furthercomonomers such as (meth)acrylamide and acetonitrile. Such copolymersare known, for example, from EP-A 51 144.

The spray-drying is preferably carried out without spray auxiliaries.

The dye-containing microcapsule dispersions obtained according to theinvention, and the microcapsule powders are characterized by a highbrilliance. They are particularly suitable for incorporation intononpolar media, for example as color-imparting component in printinginks, alkyl resin varnishes, e.g. melamine-alkyl resin stoving enamelsor for coloring plastic fibers or plastic compositions.

The dye-containing microcapsule dispersions according to the inventionand the dye-containing microcapsule powders obtainable therefrom arecharacterized, even after incorporation into high molecular weightapplication media such as varnishes, printing inks, plastics orinorganic materials, by high color intensity and high brilliance, andgood transparency. In contrast to conventional pigments, a dependency ofthe color shade on the microcapsule particle size is not observed.Moreover, in contrast to conventional pigments, the microcapsuledispersions according to the invention do not have shape anisotropy andthus associated rheology problems, and also have a narrow particle sizedistribution. In addition, the average particle size can be determinedreadily via the particle size of the emulsion droplets. Moreover, themicrocapsule dispersions according to the invention are characterized,compared with conventional pigments, by a very much lower requirement ofexpensive chromophore for achieving the same color impression.Furthermore, the dyes in the microcapsules are better protected againstbleaching as a result of the effect of UV radiation or oxygen thanconventional dyes or pigment. Substrates treated with the novelmicrocapsule dispersions comprising optical brighteners exhibit a lowertendency toward yellowing, in particular when exposed to UV radiation orunder the action of elevated temperature. In addition, where suchmicrocapsule dispersions are used, a degree of whiteness comparable withthat of conventional optical brighteners is achieved even withrelatively small amounts of optical brightener. Microcapsule dispersionsaccording to the invention which comprise optical brighteners areparticularly suitable as brightening constituent in paper coating slips.They can also be added to the paper pulps themselves as brighteningconstituent to improve the degree of whiteness.

The examples given below serve to illustrate the present invention inmore detail.

EXAMPLE 1

In a cylindrical 2 l stirred vessel, 16.4 g of an orange dye, dissolvedin a clear aqueous solution, adjusted to pH 4.5 using formic acid, of115 g of partially methylated precondensate (comprises 2.3 CH₃O groupsper melamine molecule) of 1 mol of melamine and 5.25 mol of formaldehydein 229 g of ethylene glycol and 63.45 g of water (70:30) were dispersedin a solution of 75 g of protective colloid D (described in DE-A3709921, p. 10) in 548 g of cyclohexane in a dispersion apparatus(®Turrax 45 N, Jahnke & Kunkel) at a rotary speed of 8 000 rpm. Theresulting emulsion was heated to 60° C. at a stirring speed of 1 000rpm. After 120 min, the temperature was increased to 70° C. After afurther 90 min, the dispersion was cooled to room temperature.

The resulting dispersion was orange-milky and, according to microscopicassessment, contained individual capsules of predominantly 1 to 5 μm indiameter. The viscosity was 4.02 mPas at a shear rate of 250 l/s and asolids content of 38.5% by weight.

EXAMPLE 2

A microcapsule dispersion was prepared according to example 1. The dyeused was 16.4 of a green dye. The dye was dissolved together with 115 gof partially methylated precondensate in 292 g of water which had beenadjusted to pH 4.5 with formic acid. The resulting dispersion wasmilky-green and, according to microscopic assessment, containedindividual capsules predominantly 1 to 5 μm in diameter. The viscositywas 8.04 mPas at a shear rate of 250 l/s and a solids content of 38.5%by weight.

The given viscosities (mPas) were determined in accordance with ISO 3219(DIN 53019) using a Physica MC20 viscometer in the Z1 measuring system.The capsule diameter was determined optically at 400× magnificationusing a microscope from Leitz (Diaplan 101/107).

EXAMPLE 3

200 g of the dispersion from example 1 were dried in a laboratory spraydryer (#190 from Büchi) at 70° C. and a delivery rate of 300 g/h. Thisgave 61 g of an orange-colored powder.

To test the capsule quality, 5 g of microcapsule powder were dispersedin 100 ml of water at a stirring rate of 200 rpm and stirred for 90 minat 20° C. Following sedimentation of the powder, no coloration of thewater is discernible.

EXAMPLE 4

A powder was prepared from the dispersion described in example 2analogously to example 3. 200 g of dispersion gave 58 g of a greenpowder. The test analogous to example 3 for imperviousness against waterlikewise revealed, following sedimentation of the green powder, nodiscernible coloration of the water.

1. A process for the preparation of microcapsules having a capsule corecomprising water-soluble organic substances as solution in ahydrophillic solvent, and a capsule coating which is a condensate offormaldehyde resins and/or alkyl ethers thereof, by condensingformaldehyde resins in the hydrophilic phase of a water-in-oil emulsionin the presence of a protective colloid.
 2. The process for thepreparation of microcapsules as claimed in claim 1, wherein the capsulecoating is a condensate of melamine-formaldehyde resins and/or methylethers thereof.
 3. The process for the preparation of microcapsules asclaimed in claim 1, wherein the capsule core comprises water-solubledyes as water-soluble organic substances.
 4. The process for thepreparation of microcapsules as claimed in claim 1, wherein its averageparticle size is 0.5 to 50 μm.
 5. The process for the preparation ofmicrocapsules as claimed in claim 1, wherein the protective colloid is agraft polymer obtained by grafting polymers (A) comprising: a) 40 to 95%by weight of monovinylaromatic monomers; b) 5 to 60% by weight ofmonoethylenically unsaturated carboxylic acids having 3 to 6 carbonatoms, maleic anhydride and/or itaconic anhydride; and c) 0 to 20% byweight of other monoethylenically unsaturated monomers, with the provisothat the sum of the percentages by weight (a) to (c) is always 100 andthe polymers (A) have a molecular weight (number-average) of from 500 to20 000, with monomer mixtures of: a) 70 to 100% by weight of acrylicesters and/or methacrylic esters of monohydric alcohols containing 1 to20 carbon atoms; b) 0 to 15% by weight of monoethylenically unsaturatedcarboxylic acids having 3 to 6 carbon atoms, maleic anhydride and/oritaconic anhydride; c) 0 to 10% by weight of acrylic monoesters and/ormethacrylic monoesters of at least dihydric alcohols; d) 0 to 15% byweight of monovinylaromatic monomers; and e) 0 to 7.5% by weight ofacrylamide and/or methacrylamide with the proviso that the sum of thepercentages by weight a) to e) is always 100, where the monomers areused in an amount of from 50 to 97.5% by weight, based on the mixture ofpolymers (A) and monomers.
 6. The process for the preparation ofmicrocapsules as claimed in claim 1, wherein the condensation is carriedout in the temperature range from 40 to 150° C.
 7. A microcapsuleobtained by a process as claimed in claim
 1. 8. A printing ink, alkylresin paint, plastic fiber or plastic composition comprisingmicrocapsules as claimed in claim 7 in the form of a polymer dispersionor a polymer powder.
 9. A printing ink, alkyl resin paint, plastic fiberor plastic composition comprising microcapsules as claimed in claim 3 inthe form of a polymer dispersion or a polymer powder as acolor-imparting constituent.
 10. A plastic composition comprising 0.1 to50% by weight, based on the total weight of the plastic composition, ofmicrocapsules as claimed in claim 7 comprising water-soluble dyes andadditives customary for plastic compositions.